1. Field of the Present Invention
The present invention relates to a processing method for motion measurement, and more particularly to a processing method for a motion inertial measurement unit, wherein output signals of an angular rate producer and acceleration producer, such as an angular rate device array and an acceleration device array, or an angular rate and acceleration simulator, are processed to obtain highly accurate attitude and heading measurements of a carrier under dynamic environments.
2. Description of Related Arts
Generally, conventional methods for determining the motion of a carrier are to employ inertial angular rate devices and acceleration devices, such as gyros and accelerometers, radio positioning systems, and hybrid systems.
In principle, inertial motion measurement methods depend on three orthogonally mounted inertial rate sensors and three orthogonally mounted accelerometers to obtain three-axis rate and acceleration measurement signals. The three orthogonally mounted inertial rate sensors and three orthogonally mounted accelerometers with additional supporting mechanical structure and electronics devices are conventionally called an Inertial Measurement Unit (IMU). The existing IMUs may be cataloged into Platform IMU and Strapdown IMU.
In the Platform IMU, rate sensor and accelerometers are installed on a stabilized platform. Attitude measurements can be directly picked off from the platform structure. But attitude rate measurements can not be directly obtained from the platform. Moreover, there are highly accurate feedback controlling loops associated with the platform.
Compared with the platform IMU, in the strapdown IMU, rate sensors and accelerometers are directly fixed in the carrier and move with the carrier. The output signals of strapdown rate sensors and accelerometers are expressed in the carrier body frame. The attitude and attitude rate measurements can be obtained by means of a series of computations.
Conventional inertial rate sensors include Floated Integrating Gyros (FIG), Dynamically Tuned Gyros (DTG), Ring Laser Gyros (RLG), Fiber-Optic Gyros (FOG), Electrostatic Gyros (ESG), Josephson Junction Gyros (JJG), Hemisperical Resonating Gyros (HRG), etc.
Conventional accelerometer includes Pulsed Integrating Pendulous Accelerometer (PIPA), Pendulous Integrating Gyro Accelerometer (PIGA), etc.
The processing methods in conventional IMUs vary with types of gyros and accelerometers used in the IMUs. Because conventional gyros and accelerometers have big size, large power consumption, and moving mass, complex feedback controlling loops are required to obtain stable motion measurements. For example, dynamic-tuned gyros and accelerometers need force-rebalance loops to create a moving mass idle position. There are often pulse modulation force-rebalance circuits associated with dynamic-tuned gyros and accelerometer based IMUs.
New horizons are opening up for inertial sensor device technologies. MEMS (MicroElectronicMechanicalSystem) inertial sensors offer tremendous cost, size, reliability improvements for guidance, navigation, and control systems, compared with conventional inertial sensors. It is well-known that the silicon revolution began over three decades ago, with the introduction of the first integrated circuit. The integrated circuit has changed virtually every aspect of our lives. The hallmark of the integrated circuit industry over the past three decades has been the exponential increase in the number of transistors incorporated onto a single piece of silicon. This rapid advance in the number of transistors per chip leads to integrated circuits with continuously increasing capability and performance. As time has progressed, large, expensive, complex systems have been replaced by small, high performance, inexpensive integrated circuits. While the growth in the functionality of microelectronic circuits has been truly phenomenal, for the most part, this growth has been limited to the processing power of the chip.
MEMS, or, as stated more simply, micromachines, are considered the next logical step in the silicon revolution. It is believed that this coming step will be different, and more important than simply packing more transistors onto silicon. The hallmark of the next thirty years of the silicon revolution will be the incorporation of new types of functionality onto the chip structures, which will enable the chip to, not only think, but to sense, act, and communicate as well.
Prolific MEMS angular rate sensor approaches have been developed to meet the need for inexpensive yet reliable angular rate sensors in fields ranging from automotive to consumer electronics. Single input axis MEMS angular rate sensors are based on either translational resonance, such as tuning forks, or structural mode resonance, such as vibrating rings. Moreover, Dual Input Axis MEMS Angular Rate Sensors may be based on angular resonance of a rotating rigid rotor suspended by torsional springs. The inherent symmetry of the circular design allows angular rate measurement about two axes simultaneously. Preferred MEMS angular rate sensors are mostly based on an electronically-driven tuning fork method. Such MEMS gyros operate in accordance with the dynamic theory (Coriolis Effect) that when an angular rate is applied to a translating body, a Coriolis force is generated. When this angular rate is applied, the axis of an oscillating tuning fork, its tines receive a Coriolis force, which then produces torsional forces about the sensor axis. These forces are proportional to the applied angular rate, which then can be measured capacitively, as shown in FIG. 16.
MEMS devices can be fabricated by bulk micromachining (chemical etching) single crystal silicon or by surface micromachining layers of ploysilicon. Surface micromachined devices are typically a few microns to 10 microns thick while bulk machining produces devices 100 to 500 microns thick. Angular rate sensors created with surface machining have very low masses and are presently not sensitive enough for military applications but are useful for automotive applications. Bulk machining produces devices with far greater mass but it is a much more expensive technology. Allied Signal produces bulk machined inertial sensors. The advantage of surface machining is the low cost and the ease of incorporating the electronics close to the sensor.
FIG. 17 depicts the basis of the Charles Stark Draper Laboratory design based on an electronically-driven tuning fork method. The MEMS angular rate sensor measures angular rate voltage signals by picking-off a signal generated by an electromechanical oscillating mass as it deviates from its plane of oscillation under the Coriolis force effect when submitted to a rotation about an axis perpendicular to the plane of oscillation. Two vibrating proof masses are attached by springs to each other and to the surrounding stationary material. The vibrating (dither) proof masses are driven in opposite directions by electrostatic comb drive motors to maintain lateral in-plane oscillation. The dither motion is in the plane of the wafer. When an angular rate is applied to the MEMS device about the input axis (which is in the plane of the tines), the proof masses are caused to oscillate out of plane by a Coriolis force due to Coriolis effect. The resulting out-of-plane up and down oscillation motion amplitude, proportional to the input angular rate is detected and measured by capacitive pickoff plates underneath the proof masses. The device can either be designed for closed loop or open loop operation. Running the device closed loop adds more complexity but less cross coupling and better linearity. The comb drives move the masses out of phase with respect to each other. The masses will then respond in opposite directions to the Corilois force.
Several MEMS accelerometers incorporate piezoresistive bridges such as those used in early micromechnical pressure gauges. More accurate accelerometers are the force rebalance type that uses closed-loop capacitive sensing and electrostatic forcing. Draper""s micromechnical accelerometer is a typical example, where the accelerometer is a monolithic silicon structure consisting of a torsional pendulum with capacitive readout and electrostatic torquer. Analog Device""s MEMS accelerometer has interdigitated ploysilicon capacitive structure fabricated with on-chip BiMOS process to include a precision voltage reference, local oscillators, amplifiers, demodulators, force rebalance loop and self-test functions.
Analog Device""s MEMS accelerometer is a combination of springs, masses, motion sensing and actuation cells. It consists of a variable differential air capacitor whose plates are etched into the suspended polysilicon layer. The moving plate of the capacitor is formed by a large number of xe2x80x9cfingersxe2x80x9d extending from the xe2x80x9cbeamxe2x80x9d, a proof mass supported by tethers anchored to the substrate. Tethers provide the mechanical spring constant that forces the proof mass to return to its original position when at rest or at constant velocity, as shown in FIG. 18, which shows a micromachined sensor unit. The fixed plates of the capacitor are formed by a number of matching pairs of fixed fingers positioned on either side of the moving fingers attached to the beam, and anchored to the substrate.
When responding to an applied acceleration or under gravity, the proof mass"" inertia causes it to move along a predetermined axis, relative to the rest of the chip, as shown in FIG. 19. As the fingers extending from the beam move between the fixed fingers, capacitance change is being sensed and used to measure the amplitude of the force that led to the displacement of the beam.
To sense the change in capacitance between the fixed and moving plates, two 2 MHz square wave signals of equal amplitude, but 180xc2x0 out of phase from each other, are applied to the fingers forming the fixed plates of the capacitor. At rest, the space between each one of the fixed plates and the moving plate is equidistant, and both signals are coupled to the movable plate where they subtract from each other resulting in a waveform of zero amplitude.
As soon as the chip experiences acceleration, the distance between one of the fixed plates and the movable plate increases while the distance between the other fixed plate and the movable plate decreases, resulting in capacitance imbalance. More of one of the two square wave signals gets coupled into the moving plate than the other, and the resulting signal at the output of the movable plate is a square wave signal whose amplitude is proportional to the magnitude of the acceleration, and whose phase is indicative of the direction of the acceleration.
The signal is then fed into a buffer amplifier and further into a phase-sensitive demodulator (synchronized on the same oscillator that generates the 1 MHz square wave excitation signals), which acts as a full wave-rectifier and low pass filter (with the use of an external capacitor). The output is a low frequency signal (dc to 1 kHz bandwidth), whose amplitude and polarity are proportional to acceleration and direction respectively. The synchronous demodulator drives a preamplifier whose output is made available to the user.
FIG. 20 shows the silhouette of the sensor structure used in Analog Device""s MEMS accelerometer, ADXL 50. The microscopic sensor structure is surrounded by signal conditioning circuitry on the same chip. The sensor has numerous fingers along each side of the movable center member; they constitute the center plates of a parallel set of differential capacitors. Pairs of fixed fingers attached to the substrate interleave with the beam fingers to form the outer capacitor plates. The beam is supported by tethers, which serve as mechanical springs. The voltage on the moving plates is read via the electrically conductive tether anchors that support the beam.
A main objective of the present invention is to provide a processing method of motion measurement which successfully incorporates the MEMS technology with the IMU industry.
Another objective of the present invention is to provide a processing method of motion measurement which is adapted to be applied to output signals of rate sensors and acceleration sensors, which are proportional to rotation and translational motion of the carrier, respectively, and more suitable for emerging MEMS (MicroElectronicMechanicalSystem) rate and acceleration sensors. Compared with a conventional IMU, the present invention utilizes a feedforward open-loop signal processing scheme to obtain highly accurate motion measurements by means of signal digitizing, temperature control and compensation, sensor error and misalignment calibrations, attitude updating, and damping controlling loops, and dramatically shrinks the size of mechanical and electronic hardware and power consumption, meanwhile, obtains highly accurate motion measurements.
Another objective of the present invention is to provide a processing method for outputting voltage signals of an angular rate producer and an acceleration producer, such as an angular rate device array and acceleration device array, or an angular rate and acceleration simulator, are processed to obtain digital highly accurate digital angular increment and velocity increment measurements of the carrier, and are further processed to obtain highly accurate attitude and heading measurements of the carrier under dynamic environments.
Although the present invention can be applicable to existing angular rate devices and acceleration devices, the present invention is specifically suitable for emerging MEMS angular rate devices and acceleration devices assembled into a core micro IMU, wherein the core micro IMU has the following unique features:
(1) Attitude Heading Reference System (AHRS) Capable Core Sensor Module.
(2) Miniaturized (Length/Width/Height) and Light Weight.
(3) High Performance and Low Cost.
(4) Low Power Dissipation.
(5) Dramatic Improvement In Reliability (microelectromechanical systems MEMS).
Another objective of the present invention is to provide a processing method for motion measurement which enables the core micro IMU rendering into an integrated micro land navigator that has the following unique features:
(1) Miniature, light weight, low power, low cost.
(2) AHRS, odometer, integrated GPS chipset and flux valve.
(3) Integration filter for sensor data fusion and zero velocity updating.
(4) Typical applications: automobiles, railway vehicles, miniature land vehicles, robots, unmanned ground vehicles, personal navigators, and military land vehicles.
Another objective of the present invention is to provide a processing method for motion measurement which enables the core micro IMU to function as aircraft inertial avionics, which has the following unique features:
(1) Rate Gyro
(2) Vertical Gyro
(3) Directional Gyro
(4) AHRS
(5) IMU
(6) Inertial Navigation System
(7) Fully-Coupled GPS/MEMS IMU Integrated System
(8) Fully-Coupled GPS/IMU/Radar Altimeter Integrated System
(9) Universal vehicle navigation and control box.
Another objective of the present invention is to provide a processing method for motion measurement which enables the core micro IMU to function as a Spaceborne MEMS IMU Attitude Determination System and a Spaceborne Fully-Coupled GPS/MEMS IMU Integrated system for orbit determination, attitude control, payload pointing, and formation flight, that have the following unique features:
(1) Shock resistant and vibration tolerant
(2) High anti-jamming
(3) High dynamic performance
(4) Broad operating range of temperatures
(5) High resolution
(6) Compact, low power and light weight unit
(7) Flexible hardware and software architecture
Another objective of the present invention is to provide a processing method for motion measurement which enables the core micro IMU to form a marine INS with embedded GPS, which has the following unique features:
(1) Micro MEMS IMU AHRS with Embedded GPS
(2) Built-in CDU (Control Display Unit)
(3) Optional DGPS (Differential GPS)
(4) Flexible Hardware and Software System Architecture
(5) Low Cost, Light Weight, High Reliability
Another objective of the present invention is to provide a processing method for motion measurement which enables the core micro IMU to be used in a micro pointing and stabilization mechanism that has the following unique features:
(1) Micro MEMS IMU AHRS utilized for platform stabilization.
(2) MEMS IMU integrated with the electrical and mechanical design of the pointing and stabilization mechanism.
(3) Vehicle motion, vibration, and other interference rejected by a stabilized platform.
(4) Variable pointing angle for tracker implementations.
(5) Typical applications: miniature antenna pointing and tracking control, laser beam pointing for optical communications, telescopic pointing for imaging, airborne laser pointing control for targeting, vehicle control and guidance.